Optical fibers for use in optical communication are usually fine fibers made of a glass, quartz-based single mode optical fibers for use in a long distance optical communication being composed, for example, of a core portion with an outer diameter of about 10 .mu.m and a clad portion with an outer diameter of 125 .mu.m for coating the core portion. Quartz-based multi-mode optical fibers are composed of a core portion with an outer diameter of 50 to 125 .mu.m and a clad portion with an outer diameter of 125 .mu.m for coating the core portion. Accordingly, when optical fibers are optically connected with each other (the phrase "optically connect" is referred to "optical interconnection" hereinafter) or optical fibers are optically connected with optical element(s) such as an optical waveguide, a lens, an light emitting element and a light acceptance element, a highly precise alignment is required for reducing connection loss at the optical interconnection part. Especially, in the optical interconnection of the quartz-based single mode optical fibers with each other and optical interconnection of the quartz-based single mode optical fiber with a quartz-glass-based single mode optical waveguide, an alignment with an accuracy of as high as about .+-.1 mm is required.
When optical fiber(s) is/are optically connected with other optical fiber(s) or optical element(s), the optical fiber(s) is/are previously fixed in an optical fiber fixing tool such as an optical connector or optical fiber array. The optical fiber array as used herein is at least provided with an optical fiber guide block comprising a thin plate on which optical fiber fixing engagement portion(s) for fixing (positioning) the optical fiber(s) is/are formed, optical fiber(s) engaged to the optical fiber fixing engagement portion(s) and a holding block comprising a thin plate for compression-fixing optical fiber(s) engaged to the optical fiber fixing engagement portion(s).
For example, Japanese Unexamined Patent Publication No. Hei 7-5341 discloses an optical fiber array fixing a tape fiber produced by protecting given strings of optical fiber disposed in parallel relation with each other with a coating material comprising a resin. As shown in FIG. 26, the optical fiber array 200 disclosed in the foregoing patent publication is provided with a thin plate of an optical fiber guide block 204 on which a given number of V-shaped grooves 203 as optical fiber fixing engagement portions for fixing optical fibers 202 exposed from a tape fiber 201, optical fibers 202 and a thin plate of an optical fiber holding block 205 for compression-fixing the optical fiber 202 engaged to the V-shaped grooves 203 described above. The optical fiber guide block 204 constituting this optical fiber array 200 has a pedestal 207 for fixing the coated optical fiber 206 in the tape fiber 201 besides the V-shaped grooves 203 described above, the pedestal 207 being formed by one step lower than the V-shaped grooves 203. The optical fiber array 200 is also provided with a coated optical fiber holding block 208 with a given cross sectional configuration for compression-fixing the coated optical fiber 206 fixed on the pedestal 207.
An active alignment using a precision stage has been applied in the optical interconnection of the optical fibers fixed in optical fiber fixing tools such as an optical connector or optical fiber array with each other, or in the optical interconnection of the optical fiber(s) fixed in the optical fiber fixing tool and optical element(s) with a high alignment accuracy as described previously. This active alignment is executed, for example, in the mutual connection of the optical fibers fixed in optical fiber arrays as follows.
After fixing one optical fiber array (referred to "optical fiber array A" hereinafter) on which optical fiber(s) is/are fixed to one holder on the precision stage, the other optical fiber array (referred to "optical fiber array B" hereinafter) on which optical fiber(s) is/are fixed is fixed to the other holder on the precision stage. Then, a light beam is allowed to irradiate the optical fiber fixed with the optical fiber array A from the tip of the optical fiber, positioned at an opposite end to the optical end face (of the side faces of an optical fiber array, a side face positioned at the optical interconnection side where the optical fiber array is connected to the other optical fiber array or optical element: the same hereinafter). An optical detector is provided at the end of the optical fiber positioned at the opposite side of the optical end face in the optical fiber array B. The precision stage is then scanned in a wide range to search the position where any faint optical power is detected with the optical detector (this stage is referred to "first step"). The precision stage is finely scanned thereafter so that the optical detector senses the maximum optical power level, thereby completing the desired high accuracy alignment (this stage is referred to "second step").
Since a long period of time is required for wide range scanning at the first step of the active alignment described above, it is desirable that the first step is substantially completed at the stage when the optical fiber fixing tools are fixed on the holders described above for making a highly precise alignment easy. Accordingly, it is desirable to precisely adjust the locational accuracy of the optical fiber fixing engagement portion(s), measured with reference to the bottom face or side face of the optical fiber fixing tool, to a degree of about 1/1 or less of the core diameter of the optical fiber fixed in the optical fiber fixing tool, along with precisely adjusting the dimensional accuracy of the optical fiber fixing engagement portion(s) for fixing optical fiber(s) in the optical fiber fixing tool. In the optical interconnection of the quartz-based single mode optical fibers with a core diameter of about 10 .mu.m with each other or in the optical interconnection between the quartz-based single mode optical fiber and quartz-glass-based single mode optical waveguide, for example, the locational accuracy is desirably 10 .mu.m or less and the alignment will be made more easily when this locational accuracy is 5 .mu.m or less.
Optical interconnection by a passive alignment is made possible by adjusting the locational accuracy described above to about 1/10 or less of the core diameter of the optical fiber. The passive alignment refers to a method in which, with no need of irradiating an beam to optical fiber(s) or detecting an emission beam from optical fiber(s), an alignment of the optical fiber fixing tools with each other or an alignment between the optical fiber fixing tool and an optical element is carried out merely by a mechanical positioning by taking advantage of the bottom face or side face of the optical fiber fixing tool as a reference face.
The passive alignment may be possibly executed by providing alignment mark(s) with a high locational accuracy at desired position(s) of the optical fiber fixing tool, along with making use of a specified face of the optical fiber fixing tool as a reference face. Otherwise, a guide pin engagement portions with a high locational accuracy are provided at desired positions of the two members to be connected (optical fiber fixing tools with each other or an optical fiber fixing tool and an optical element) with each other, thereby the passive alignment can be carried out by an optical interconnection of the optical fibers with each other or by an optical interconnection between the optical fiber fixing tool and optical element using guide pin(s).
For the passive alignment using the alignment marks, the locational accuracy of the alignment marks is desirably adjusted to about 1/10 or less of the core diameter of the optical fiber to be optically connected, along with adjusting the dimensional accuracy of the optical fiber fixing engagement portions in the optical fiber fixing tools with a high precision. For the passive alignment using the guide pin(s), the locational accuracy of the guide pin(s) (guide pin(s) after engaging with the optical fiber fixing tools) is desirably adjusted to about 1/10 or less of the core diameter of the optical fiber to be optically connected, along with adjusting the dimensional accuracy of the optical fiber fixing engagement portions in the optical fiber fixing tools with a high precision.
Connection of the optical fiber fixing tools with each other using the guide pin(s) has been already proposed. For example, Japanese Unexamined Patent Publication No. Sho 62-269108 discloses an optical connector ferrule using guide pins. As shown in FIG. 27(a) and (b), guide pin holes (not shown) are formed on the optical connector ferrule 210 disclosed in the patent publication. In the optical interconnection of the optical connector ferrules 210 with each other after fixing the optical fibers 211 to them, one end of the guide pin 212 is inserted into guide pin hole formed on one of the ferrules 210 and two ferrules 210 are connected with each other by inserting the other end of the foregoing guide pin 212 into guide pin hole formed on the other ferrule 210. Two ferrules 210 connected with each other using the guide pins 212 are press-fixed by means of a clamp 213 and accommodated in a cylindrical housing.
Japanese Unexamined Patent Publication No. Hei 7-35958 also discloses an optical fiber array using guide pins. As shown in FIG. 28, the optical fiber array 220 disclosed in the patent publication is provided with an optical fiber guide block 222 on which a given strings of V-shaped groove 221 as optical fiber fixing engagement portions for fixing optical fibers are formed, and a holding block 223 for compression-fixing the optical fibers engaged to the V-shaped grooves 221 described above. Grooves with a given shape are provided on each optical fiber guide block 222 and holding block 223 so as to form guide pin grooves 224 opened toward the optical end face of the optical fiber array 220 when the optical fiber array 220 is assembled using these blocks.
Foregoing each optical fiber array illustrated in FIG. 26 and FIG. 28 is an optical fiber array for use in butt joint type connection by directly confronting the end face of the optical fibers with each other. In this type of optical fiber arrays, the optical end face 232 is generally polished backwardly so that an angle .theta. becomes about 6 to 45.degree., usually 8.degree., as shown in FIG. 29, in order to suppress the effect of backwardly reflected light at the optical interconnection site, said angle .theta. is an angle formed between one plane, a plane perpendicular to the optical axis of the optical fiber 231 to be optically connected, and the other face, an optical end face 232 at the optical fiber array 230.
Parts used for the members constituting the optical fiber fixing tools (referred to "optical fiber fixing member" hereinafter) such as an optical connector and optical fiber array described above include glasses, ceramics, silicon and resins. Of these optical fiber fixing member, the optical fiber guide block, in which the optical fiber fixing engagement portion(s) is required to have a high dimensional accuracy, has been produced by a mechanical processing of a glass block using a dicing saw or diamond grindstone.
While adhesion with adhesives or soldering, anode welding and heat welding have been conventionally used for fixing the optical fiber guide block and holding block or for connecting an optical fiber array with other optical fiber array or an optical element, use of ultraviolet curing adhesives is desired in recent years by the reasons of workability, etc. In accordance with this tendency, a glass having a good transmissivity to ultraviolet ray has been advantageously used for materials of the optical fiber array.
Recently, Pyrex glass (thermal expansion coefficient: 30.times.10.sup.-7 /.degree. C., "Pyrex" is a trade name of a low thermal expansion glass made by Corning Co.) has been preferably used for an optical fiber array for the optical interconnection between a quartz-glass-based (thermal expansion coefficient: 5.times.10.sup.-7 /.degree. C.) single mode optical waveguide and quartz-based single mode optical fiber by the following reasons: When the difference between the thermal expansion coefficient of a substrate of the quartz-glass-based single mode optical waveguide and that of the optical fiber array is large, the alignment accuracy will be lowered due to a positional shift caused at the optical interconnection site by a change in an ambient temperature, thereby causing an increased connection loss at the optical interconnection site, even if the quartz-glass-based single mode optical waveguide is optically connected to the quartz-based single mode optical fiber with an alignment accuracy of .+-.1 .mu.m or less. Accordingly, it is preferable that the difference between the thermal expansion coefficient of a substrate of the quartz-glass-based single mode optical waveguide and that of the optical fiber array is as small as possible. Therefore, Pyrex glass having a small thermal expansion coefficient can be advantageously used for the material of the optical fiber array to be used as described above.
However, it is a problem in producing optical fiber fixing members, especially optical fiber guide blocks made of a glass, by a mechanical processing using a dicing saw or diamond grindstone, that their production cost becomes high and mass production is difficult.
While it is relatively easy to form optical fiber fixing engagement portion(s) with a high dimensional precision in producing an optical fiber guide block made of a glass by a mechanical processing, it is difficult to improve the locational accuracy of the optical fiber fixing engagement portion(s) to a degree so as to be able to substantially omit the first step in the active alignment. When the locational accuracy of the optical fiber fixing engagement portion(s), locational accuracy of the alignment mark(s) against the optical fiber fixing engagement portion(s) and locational accuracy of the guide pin against the optical fiber fixing engagement portion(s) are required to be so high as to enable an passive alignment, it is more difficult to produce an optical fiber guide block made of a glass by a mechanical processing.
Since a knife edge is formed at the corner of the optical fiber guide block made of a glass produced by a mechanical processing, the edge is liable to be broken when the optical fiber array using the optical fiber guide block is fixed on the precision stage, when the alignment is carried out or when the optical end face of the optical fiber array using the optical fiber guide block is polished.
There has been established an art for producing a spherical or non-spherical concave or convex lens from a spherical or marble-shaped glass shaping preform by means of a mold by applying a molding (hot-molding or press molding). Therefore, application of the above molding technology may be taken into consideration for mass production of the optical fiber fixing member made of a glass.
However, when one attempts to form a thin-plate like optical fiber fixing member such as an optical fiber guide block and holding block by the molding technology of a lens as described above, a precision molding becomes very difficult because protruding of a glass (molding fin) or an insufficient transfer accuracy may be often caused. Accordingly, it is very difficult to obtain a practical optical fiber fixing member by the molding method as described above.
While protruding of a glass may be solved by using a side-free mold, that is, a mold of the type in which there is no side walls in the mold viewed along the direction of pressurizing in molding, the problem of an insufficient transfer accuracy arising from shortage of the filled glass volume can not be solved. Therefore, it is difficult to obtain an optical fiber fixing member by which an alignment with a high precision is made easy. There will be occurred another new problem that scattering of accuracy among the molded articles becomes large when such side-free mold is used.
The glass with a low thermal expansion coefficient represented by foregoing Pyrex glass has molding temperature of far more higher than 600.degree. C., so that there arise a problem that a mold and its mold release film tend to be damaged greatly when such glass is molded.